This invention relates generally to optical modulators and, more particularly, to techniques for modulation of an optical carrier with a radio-frequency communication signal. The benefits of optical transmission of high-frequency data over long-distance telecommunications systems are well established. Many of these benefits, such as immunity to electromagnetic interference, very wide bandwidth, and lightness of weight, would be of substantial value for short communication links as well, but are more difficult to obtain because of inherent inefficiencies of existing high-speed optical links.
Optical transmission of radio-frequency (rf) signals requires two major transducer components: an optical modulator to convert electrical rf signals to corresponding fluctuations in light intensity, and a demodulator, such as a photodiode, to convert the modulated optical carrier back into electrical rf signals. The invention is primarily concerned with the modulation process, wherein an electrical rf signal modulates the intensity of a light beam. In this description it will be understood that "light beam," "optical carrier" and similar terms are used to refer to radiation in the visible portion of the electromagnetic frequency spectrum, but that the principles of the invention also apply to radiation at frequencies outside the visible range, such as in the infrared or ultraviolet portions of the spectrum.
The invention is specifically concerned with optical modulation of the analog type. The modulating rf signal is continuously varying in amplitude and these variations are to be faithfully reproduced as corresponding variations in the intensity of the optical carrier. This is to be contrasted with digital optical modulation, wherein the modulating signals have only a small number of possible amplitude levels (usually two).
There are two major requirements for analog optical modulation. One is that the intensity variations in the optical signal must be a faithful reproduction of the original rf signal. In other words, the modulator must provide a linear relationship between its input and output signals. Changes in the electrical input signal are reflected in proportional changes in the optical output intensity. The other requirement is that the variations in optical intensity should be as strong as possible. If conversion of the rf signal to and from the optical form results in loss of rf signal amplitude, the electro-optical conversion components are said to result in rf insertion loss. Conventional electro-optical modulators, such as the Mach-Zehnder modulator, have a less than desirable performance in terms of both linearity and rf insertion loss. There is extensive literature on analog direct modulation using diode lasers, and analog external modulators based on interferometric approaches. Existing analog external modulators are large, and typically relatively narrow in bandwidth where greater linearity is desired, and are built with such materials and to such a physical scale that they are incompatible with integration into semiconductor substrates, such as substrates made from materials selected from Groups III-V of the periodic table.
Quantum-well electroabsorption modulators have been proposed for use as digital optical modulators. These devices are semiconductor waveguide devices whose light absorption properties at a given optical wavelength can be controlled by an electrical voltage applied to the waveguide section of the device.
It will be appreciated from the foregoing that there is a need for an optical modulator that is characterized by low rf insertion loss and low nonlinear distortion, i.e. a high degree of linearity. Such a modulator would be a key component for the efficient transmission of rf signals on optical carriers. The present invention satisfies this need.